Image non-uniformity has long been a difficult problem for most digital marking processes. Streaks are one-dimensional image defects that generally run parallel to the process direction in a printed image. They are typically caused by the undesirable non-uniform response of some components in a marking engine. Defects in the subsystems of a xerographic printer, an inkjet printer, or similar image forming system may give rise to visible streaks in a printed image. For example, photoreceptor scratches, contamination of the charger wire, non-uniform LED imager and Raster Output Scanner (ROS) spot size variations are examples of subsystem problems giving rise to rendered image streaking in a xerographic marking engine. Bands are also one-dimensional image defects that generally run perpendicular to the process direction in a printed image. They are typically caused by time-varying performance of a marking subsystem such as non-uniform velocity of the photoreceptor drive, out-of-roundness of development rolls, and wobble of the ROS polygon mirror. In a uniform patch of gray, streaks and bands may appear as a variation in the gray level. In general, “gray” refers to the optical density or area coverage value of any single color separation layer, whether the toner is black, cyan, magenta, yellow or some other color.
With reference to FIG. 1, an exemplary illustration of streaking as it may appear in a printed document is shown. An image patch 10, having a single gray level value, contains a number of exemplary streak defects 12 which run parallel to a process direction 14. Each streak defect in this example extends along a process or slow-scan direction, while the various different streak defects are adjacent to each other along the cross-process or fast-scan direction. The magnitude of the streaking or the difference in toner intensity is thus a function of cross-process position. It is to be appreciated, however, that the defects shown are only exemplary, and embodiments described herein are not limited to defects running parallel to a process direction. In various exemplary embodiments described herein as systems and methods, streaks or improper toner density regions caused by spatial non-uniformity are compensated for prior to actual printing of the document.
Spatial uniformity correction via spatially varying tone reproduction curves (sTRCs) and spatially varying multi-dimensional lookup tables (sDLUTs) has been demonstrated to be effective in compensating for monochromatic and process-color streaks, and in compensating for side-to-side non-uniformity. These methods generally map the set of spatially varying engine response curves across the page to the mean engine response to attain a spatially uniform overall printer response. However, it was found that mean density was not achievable for all positions on the page for densities close to the maximum density (Dmax), and an “Achievable Aim TRC” method was developed to address this problem for monochromatic streaks as described in U.S. Patent Application Document No. 2006/0077488. However, it has been shown that a similar problem exists for process-color streaks and sDLUTs. That is to say, the spatial mean color gamut may be unattainable by some of the local engine responses, thus rendering the compensation ineffective for particular colors on certain portions of the page.
Previous sDLUT implementations for streak compensation map the local engine response to the spatially averaged engine response. This method suffers from a limitation in its ability to compensate for streaks because the color gamut defined by this mean engine response may be unattainable at portions of the page where the local engine response curves have smaller color gamuts. Streak compensation will not be effective for out-of-gamut colors at those regions.
The present application provides a new and improved apparatus and method which overcomes the above-referenced problems and others.